A digital signal converter may form part of, for example, a switching amplifier that comprises a power output stage. The power output stage provides a power signal, which is an amplified version of the pulse width modulated signal. The power output stage may comprise, for example, two output transistors, one of which coupled between an output node of the switching amplifier and a supply voltage node on which a supply voltage is present. The other output transistor is coupled between the output node and signal ground. The one output transistor constitutes an open switch and the other output transistor constitutes a closed switch, or vice versa, depending on whether the pulse width modulated signal has a high value or a low value. Switching amplifiers are often referred to as class D amplifiers. Such amplifiers are power efficient and allow low distortion.
The pulse width modulated signal, which the digital signal converter provides, comprises a direct current component, which is typically a center value between the high value and the low value that the pulse width modulated signal may have. As a result, the power signal, which the switching amplifier provides, also comprises a direct current component, which is approximately half the supply voltage. So-called single-ended amplifier configurations require a direct current (DC) blocking capacitance in order to prevent the direct current component from reaching a load circuit for which the power signal is intended. In an audio application, the load circuit may be, for example, a loudspeaker or headphone.
Transient noise may occur when the switching amplifier is switched on or off. This is related to the direct current component in the pulse width modulated signal, which the digital signal converter provides. The direct current component causes a charging or a discharging of the DC blocking capacitance when the switching amplifier is switched on or off, respectively. This charging or discharging may cause transient noise. In an audio application, the transient noise may be in the form of, for example, an audible plop or click sound.
Transient noise may be suppressed by charging and discharging a DC blocking capacitance in a smooth manner. There are various manners of smoothly charging and discharging the DC blocking capacitance, one of which is as follows. A direct current modification signal is applied to the digital signal converter, which causes the pulse width modulated signal to comprise a sequence of pulses that smoothly charges or discharges the DC blocking capacitance, whichever is required. The direct current modification signal may be in the form of, for example, a so-called ramp, which has a start point or an end point that corresponds with signal ground.
However, a stability problem may occur when the DC blocking capacitance is charged or discharged by means of a direct current modification signal that is applied to the digital signal converter. A digital signal converter typically comprises a so-called noise shaper, which pushes noise, as it were, to a spectral region that is a relatively distant from a desired spectral region, which is associated with the digital input signal. A noise shaper is typically unstable when the noise shaper receives a digital input signal that has a minimum value or a maximum value. The minimum value may correspond with, for example, signal ground in a single-ended implementation. Consequently, a direct current modification signal that has signal ground as a start point or an end point will cause the noise shaper to become unstable. The noise shaper will then produce a noisy output signal, which will be present in the power signal of the switching amplifier.
U.S. Pat. No. 6,538,590 describes a system for suppressing transient noise in switched-mode amplifier systems. In this system, a switched-mode power amplifier comprises a complementary pair of metal oxide semiconductor field effect transistors (MOSFETs), which constitutes an amplifier switch. An additional pair of MOSFETs is provided. A common node of these MOSFETs is coupled to an output node through a resistor. The additional pair of MOSFETs is configured to drive a ramp on the output node.